This invention is a fill sensor for a paint gun such as those used in multi-color robotic paint systems on vehicle body finish lines. Robotic paint systems using automatic color changers are generally required to make frequent color changes. When a color change takes place, the paint gun is first flushed with a solvent to purge all traces of the former paint color; and the paint supply line and gun must then be refilled with paint of the new color. If the supply line and paint gun are not completely clear of solvent and full of the new paint, the initial paint to be sprayed will not be uniform, and the next paint job will be unacceptable. Therefore, sufficient paint must be introduced to the paint gun to completely purge the solvent and fill the gun with paint of the new color. When the gun is full, however, additional paint is dumped through the overflow line and is wasted. Since it is clearly desirable to minimize this waste while ensuring good paint jobs, it is important to know as accurately as possible when the paint gun is full of paint.
The usual prior art practice when introducing a new color into the supply line and paint gun has been to rely on a timer to indicate when sufficient paint has been added to completely fill the supply line and paint gun. However, short and long term fluctuations in the variables that affect actual fill time force the timer to be set for a longer time that prevents an incomplete fill in the worst case, at the cost of wasted paint at other times. A paint fill sensor responsive to an actual filled supply line and paint gun would significantly reduce or eliminate such waste, while still ensuring a proper fill.